Change Text & Add External Link on Order Received Page in WooCommerce

woocommerce-change-order-received-text2.php

<?php

add_filter('woocommerce_thankyou_order_received_text', 'woo_change_order_received_text', 10, 2 );
function woo_change_order_received_text( $str, $order ) {
    $new_str = 'We have emailed the purchase receipt to you. Please make sure to fill <a href="http://localhost:8888/some-form.pdf">this form</a> before attending the event';
    return $new_str;
}